Flexible Duct Connector is used to isolate and thereby prevent the transmission of vibrations generated by axial flow fans and blowers in airduct systems.
Ever since vibratory sound transmitted to ductwork was recognized as a problem, designers and installers have incorporated fabric sound isolators between fans/blowers and ductwork, both at the intake and discharge stations of the equipment. The shop method generally used was to form a fabric sleeve of which one end was fitted over the duct, the other end over the inlet flange of the equipment. There were several means of securing the fabric in place including draw bands, metal strips screwed over the fabric into the duct, etc. More recently shops began to clench metal strips to fabric on shop equipment, a time consuming and oftentimes leak-prone process.
Commercially available pre-assembled flexible duct connector, made on specialized roll forming machines, first appeared on the North American market more than 40 years ago. It offers an efficient, cost effective alternative to shop made connector. This product is usually produced in 100 foot coiled lengths consisting of a three inch wide strip of thin, industrial grade fabric to which three inch wide metal strips are clenched continuously to the fabric along each of it's two edges with single fold or double fold seams, yielding a side-by-side, metal-to-fabric-to-metal assembly about nine inches in width. (Occasionally the fabric width may be increased to as much as six inches to accommodate large fans and blowers). Pieces are then cut to length from a coil, formed into round, square or rectangular collars and then installed between the equipment and the corresponding ductwork.
When flexible connector is not used, especially in high velocity/high pressure systems, the noise which can be generated as the result of direct contact between duct and equipment may range from moderately objectionable to unacceptable, particularly in office buildings, hospitals and libraries, etc.
To meet this noise suppression requirement the width of the fabric portion need only be sufficient to physically separate the duct from the equipment. In theory this fabric strip could be as narrow as one inch.
There is a second requirement, however, that dictates a greater width. Blowers and fans are usually installed on vibration absorbing pads which also prevent their vibrations from being transmitted directly to the floor, walls or ceiling onto which the equipment may be installed. Depending upon the physical size and rotational speed of the equipment, a fan or blower will move away from its static position when activated, sometimes by as much as 2-3 inches, creating a misalignment between the equipment and the duct. There is therefore a need for the fabric portion of the connector, when installed, to be sufficiently wide to allow for slack in order to accommodate this displacement. It is this `slack` that creates a problem which can be explained as follows.
When flexible duct connector collars are installed allowing for slack at both inlet and discharge of fans and blowers, the slack on the discharge or positive pressure side of the equipment simply allows the fabric to `balloon` outwards. While this enlargement of the free x-sectional area at the fabric portion is not really desirable, it does not create much of an airflow efficiency problem.
The opposite is true of flexible connector collars installed between the round duct and the inlet (suction/vacuum) side of the equipment. The inlet flanges are almost always round in shape. The fabric portion of the collar invaginates (collapses inward) as the air passing through it, on its way to the equipment inlet, sucks the fabric inwards. This invagination can seriously reduce the net free X-sectional area at the fabric, creating a restriction that will reduce the fan/blower efficiency to a degree where theoretical airflow delivery calculations can no longer be met. This is especially serious in the case of axial flow fans where the propellor tips produce most of the air movement. In order to control this invagination some engineers will actually call for a shop made internal skeletal `cage` to be installed at the fabric area . . . costly and occasionally cumbersome and ineffective.